bution of the reflected light can be graphed on a histogram (figure 3). The intensity distribution for the undiffused light is wide, has a long tail, and a lower mean
than the intensity distribution for the diffused light. This
shows that polished stainless steel reflects diffused light
more consistently than undiffused light.

When the images are transformed into a binary image, the reason for the wide spread in the undiffused
intensity distribution is shown to be caused by all of the
scratches present in the photograph. In contrast, the diffused light image does not have this variation; very little
of the image is white. See figure 4.

PHONG REFLECTION MODEL

When an optic reflection model is applied, then the
mechanism for how undiffused light and diffused light
behaves becomes clear. The Phong Reflection Model is
an empirical model for light reflection. It is commonly
used in reflection calculations for computer rendering1.
It calculates the intensity of light using a number of inputs (see sidebar for details.)

Because no surface is perfectly smooth, even highlypolished stainless steel has tiny hills and valleys onits surface that affects the angle at which the light re-flects2. The smaller the angle between the vectors, themore intense the reflection. The model predicts that theintensity of the diffused light will be constant for anyviewer angle, and that undiffused light will reflect lightmore intensely in some areas when viewed at certainangles.

HARD AND SOFT LIGHT

The difference between undiffused and diffused light
on stainless steel does not end with how the light reflects off the steel. The geometry of the light source affects the appearance of the steel surface as well. When
the light source does not have an area much greater
than the object it is illuminating, then hard light is produced3 (figure 5). The light-producing region of an LED
is on the order of a square millimeter or less, which is
relatively small.

Figure 5: When the source of light is small compared to the illuminated object, the light produced is hard. When the source of light is large
compared to the illuminated object, the light produced is soft4. Note
the differences in the shadow patterns on the subjects.

When hard light hits an object, shadowing and masking occur4 (figure 6). Both of these phenomena are the
result of the geometry of the surface blocking light from
reaching certain areas of the surface, called shadowing,
or reflecting light in a way that regions receive extra
light covering certain areas of the surface, called masking. In contrast, soft light does not produce shadowing